I think Bucky knows that if Steve returns Stones in the correct order, he could stop his journey in 1970 or even earlier and ride forward the usual way and then hop some sort of multidimensional portal. Which, is how the Pym Particles sort of worked, by Quantum placket?

Since the multiverse contains an infinity of Peggys, that some Steves get to be happy with her doesn't negate Peggy's husband recounted in the Smithsonian Exhibit. Some of the Steves probably get to be a second husband.